You have just got to love people using non-standard headers...

core/src/main/java/jenkins/model/Jenkins.java

@@ -1923,8 +1923,23 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
         String scheme = getXForwardedHeader(req, "X-Forwarded-Proto", req.getScheme());
         buf.append(scheme).append("://");
         String host = getXForwardedHeader(req, "X-Forwarded-Host", req.getServerName());
-        buf.append(host);
+        int index = host.indexOf(':');
         int port = req.getServerPort();
+        if (index == -1) {
+            // Almost everyone else except Nginx put the host and port in separate headers
+            buf.append(host);
+        } else {
+            // Nginx uses the same spec as for the Host header, i.e. hostanme:port
+            buf.append(host.substring(0, index));
+            if (index + 1 < host.length()) {
+                try {
+                    port = Integer.parseInt(host.substring(index + 1));
+                } catch (NumberFormatException e) {
+                    // ignore
+                }
+            }
+            // but if a user has configured Nginx with an X-Forwarded-Port, that will win out.
+        }
         String forwardedPort = getXForwardedHeader(req, "X-Forwarded-Port", null);
         if (forwardedPort != null) {
             try {